feat(playground): save & load decks from the filesystem

[tilucasoli]

Closes #88.

Gives the playground editor a real .md file behind it, backed by continuous auto-save, so edits survive refresh/relaunch. A deck is now a file the user owns, living in ~/Documents/SuperDeck/. Desktop-only (macOS); the storage interface leaves room for a web impl later.

What's included

Storage layer (playground/lib/core/data/data_sources/) — mirrors the existing AssetCacheStore interface + conditional-import native-impl pattern:

• DeckFileStore (_base/_io/_stub + conditional export): read/write/createDeck/exists/pickDeckFile/watch/decksDirectoryPath. Native impl uses dart:io + path_provider + file_picker; watching reuses core's FileWatcher. Web stub throws.
• AppSettingsStore: persists the single last-opened path as JSON in the app-support dir (no new shared_preferences dependency).

Binding controller — features/editor/domain/stores/deck_file_controller.dart (ChangeNotifier):

• Debounced auto-save (~400ms); no manual save, no dirty flag.
• File watching with self-write filtering via content hash (generateValueHash) so auto-save doesn't loop.
• External wins → auto-reload when the bound file changes on disk.
• Delete/move → unbind + warn, keeping in-memory content, never resurrecting the file.
• new / open / initialize (reopen last file, fall back to a seeded default deck).

Editor integration — TextEditorController gains an onMarkdownChanged sink (from its existing single chokepoint); reloads drive the existing MarkdownEditor.replaceMarkdown port. New EditorBootstrap awaits initialize() before mounting, seeding the editor with the loaded content. Store/settings are injected via AppProviders so tests supply in-memory fakes.

UI — editor header (filename + New/Open + unbound warning banner), new-deck dialog with collision re-prompt, and ⌘N / ⌘O shortcuts.

Acceptance criteria

• Editing auto-saves to the bound .md (debounced); no manual save action.
• New (⌘N) prompts for a name, creates <name>.md, seeds a one-slide starter, rebinds; collisions re-prompt.
• Open (⌘O) opens a .md from anywhere and rebinds.
• Current filename shown in the editor header.
• On launch, last-opened file reopened; falls back to a default on first run / if missing.
• Bound file watched; the app's own auto-saves filtered from external-change detection.
• External change auto-reloads into the editor.
• Deleting/moving the bound file unbinds + warns without losing in-memory content or resurrecting the file.
• File operations behind a storage interface with a native-only impl.

Impacted packages

packages/playground only. Adds path_provider + file_picker deps (and two test-only platform-interface deps). GeneratedPluginRegistrant.swift updated to register file_picker on macOS.

Verification

• fvm flutter analyze — clean.
• fvm flutter test — 114 tests pass, incl. 13 controller-logic tests (debounce, self-write filtering, external reload without looping, delete→unbind, new/open, collision) and 6 native-store I/O tests.
• fvm flutter build macos --debug — succeeds.

Not yet done: a manual click-through of the flows in the running desktop app.

🤖 Generated with Claude Code

[github-actions]

Visit the preview URL for this PR (updated for commit 5205063):

https://superdeck-dev--pr89-tilucasoli-save-load-7n0gtvwj.web.app

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